Temporal and Azimuthal Variability in the Io Plasma Torus

Andrew StefflPeter DelamereFran Bagenal

August 9, 2005

The Cassini UVIS Io torus dataset

UVIS observing geometry November 12, 2000

UVIS observations November 12, 2000

Temporal Variations in the Io Plasma Torus

Torus Ion Composition vs. Time

Tvashtar erupted between Feb. 2000 (I24) and Dec. 2000 (G28)

Tvashtar

PrometheusPrometheus

G28 December 2000 (Enhanced Color)C21 July 1999 (“Natural” Color)

Images courtesy NASA/JPL-Caltech

Galileo Dust Detector data from Krüger et al. 2003

Cassini UVIS data collected from October 1st onwards

Model neutral source—Gaussian peaking 25 days before October 1 x3.5 increase

• Extend the torus chemistry model of Delamere et al. [2004]• Model includes:

– Electron impact ionization e.g. S + e- → S+ + 2e-

– Recombination e.g. – Charge exchange e.g. or– Radiative cooling e.g. S++ + e- → S++ + e- + ν– Coulomb collisions between the ion and electron populations

• Energy from pickup ions alone can’t produce the observed torus composition– Need an additional energy source– Add small population (~0.23% of total Ne) of hot electrons (~50 eV)

• Five basic model parameters:– Neutral source rate SN

– O/S neutrals ratio O/S– Fraction of hot electrons fh

– Temperature of hot electrons Th

– Radial transport rate τ

Modeling torus chemistry

Time Variable Torus Chemistry Model

• Allow neutral source (SN) to vary with time– Assume source increase has a Gaussian profile

• Fit for the amplitude (αN) and width (σN) of the Gaussian

• Center Gaussian increase (t0,N) on 5 Sept 2000 based on Galileo Dust Detector profile

– Transport rate proportional to 1/SN

• Model profiles can not match UVIS results unless hot electron fraction (fh) also varies with time– Assume hot electron increase is also Gaussian

– Fit for the amplitude (αh), width (σh), and center of the Gaussian (t0,h)

• O/S ratio and hot electron temperature (Th) are held constant

Comparison of Observed and Model Composition

Azimuthal Variations and

Torus Periodicities

Long-term variations with System III longitude are small…

But, significant azimuthal variations are present in the UVIS data

Short-term Azimuthal Variability

UVIS data show anti-correlation of S II and S IV

Phase vs. Time

Lomb-Scargle Periodogram Peaks at 10.07 hours

Azimuthal Variability (amplitude)

#1

#2

#3

Amplitude vs. Time

#1 #2 #3

Sinusoidal fit to derived mixing ratio Data

Modeling Azimuthal Variability

• Extend the torus chemistry model of Delamere et al. (2004) by including– 24 azimuthal bins

– Azimuthal transport of plasma• Plasma rotation speed is independent of hot electrons

• Can be fixed in System III or allowed to vary (3 km/s)

– Latitudinal averaging i.e. plasma on the centrifugal equator is offset from neutrals on the rotational equator

• To get observed modulation, 2 periods are required– Add 2 azimuthally varying hot electron sources:

• Primary hot electron (~55 eV) source rotates with 10.07 hour period

• Secondary hot electron source remains fixed in System III longitude

Conclusions• A major volcanic event occurred on Io in September 2000

– Resulted in ~3.3x increase in amount of neutrals supplied to the torus– UVIS observed the torus returning to more “typical” conditions

• The Io torus exhibits significant azimuthal variations in ion composition – Long-term variations with System III longitude only ~5%– Variations of up to 25% seen on timescales of a few days– The Io torus always shows azimuthal variation in composition– Azimuthal variations lag System III rotation period by 1.4%– Amplitude of azimuthal compositional variation appears modulated by

the pattern’s location in System III longitude• Torus chemistry models can match observed torus behavior by

including:– 3.3x increase in neutral source around September 2000– Azimuthally varying source of hot electrons that rotates 1.5% slower– Azimuthally varying source of hot electrons fixed in System III

Unanswered Questions• Why does the torus exhibit periodicity at 10.07 hours?

– What happened to the System IV periodicity at 10.21 hours? • Perhaps the observed 10.07 hour periodicity is the same phenomenon as System IV,

just at a different period.

– Is the 10.07 hour period related to the neutral source event that preceded the Cassini flyby?

– Does the torus currently exhibit a 10.07 hour periodicity? System IV? Something else?

• What mechanism(s) produces the System III-fixed and subcorotating sources of hot electrons?– Perhaps not too difficult to produce hot electrons that are fixed in System III– It’s not obvious how to produce a source of hot electrons that slips relative to

both System III and the underlying torus plasma.

• Is there some other way to reproduce the UVIS observations without two azimuthally varying sources of hot electrons?– Azimuthally-varying plasma rotation speed can’t do it.– Azimuthally-varying radial transport timescale can’t do it.